1660s, "thrust forth, push out," from Latin exertus/exsertus, past participle of exerere/exserere "thrust out, put forth," from ex "out, from within" (see ex-) + serere "attach, join; arrange, line up" (from PIE root *ser- (2) "to line up"). Meaning "put into use" is 1680s. Related: Exerted; exerting.
word-forming element attached to verbs, making nouns of state, condition, or action, from French -ion or directly from Latin -ionem (nominative -io, genitive -ionis), common suffix forming abstract nouns from verbs.
<a href="https://www.etymonline.com/word/exertion">Etymology of exertion by etymonline</a>
Harper, D. (n.d.). Etymology of exertion. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/exertion